The DOCSIS (Data Over Cable Service Interface Specification) standards define a protocol governing the transmission of data over hybrid fiber-coax networks. In contrast to the broadcast model used in traditional analog television services, many modern communications applications supported by DOCSIS networks require two-way data transfer.
As described in more detail hereinafter, a modern cable network consists of two main classes of devices: Cable Modems (CM) and Cable Modem Termination Systems (CMTS). The CM is a low-cost, mass-produced device that is used by each customer to connect to the network, while the CMTS is a large and complex piece of equipment residing on the cable operator's premises. Traffic flowing from the CMTS to the CMs is known as downstream traffic. In contrast, the transfer of data from a single CM to the CMTS is referred to as upstream traffic.
Due to the recent explosion of two-way communications applications such as voice-over-IP and video conferencing, there is significant competition for bandwidth on the upstream channels. A time division multiple access scheme is generally used to share the upstream bandwidth between the multitude of CMs present on the network. Consequently, the data flowing on the upstream channels consists of a stream of short packets, each of which may be from a different CM. In order to properly receive these fragmented upstream transmissions, a burst demodulator is required in the CMTS.
In general, the task of the receiver in a communication system is complicated by a number of physical layer distortions which are commonly present in the communication signal. Such distortions generally arise from a lack of synchronization between transmitter and receiver, nonidealities in the transmission medium, or mismatches between the hardware components used to construct the transmitter and receiver. Unless compensated for in the receiver, these distortions tend to impair the performance of the communication system.
Accordingly, when a CM initially connects to the cable network, it is necessary for the upstream demodulator in the CMTS to estimate and correct for a number of physical layer parameters, including symbol timing error, carrier frequency error, symbol rate error, and channel attenuation. As is common in burst communication systems, DOCSIS upstream packets begin with a known preamble in order to facilitate this process. The preamble of a DOCSIS upstream packet is followed by a variable-length data segment, which contains the payload of the transmission.
In DOCSIS systems, the CMTS is responsible for dynamically defining the content and length of the preamble on a CM-by-CM basis. The receiver algorithms used during the synchronization process determine the number of preamble symbols required in each packet, thus having a significant impact upon the overall efficiency of the upstream channels.
The problem of estimating the carrier frequency of a burst digital signal has been well-studied over the last half-century or so. Rife and Boorstyn laid the groundwork for the field (as set out in Document 7 below) by deriving the Cramer-Rao bounds (CRB) and maximum likelihood estimators for the estimation of the amplitude, frequency, and phase of a single tone from discrete-time observations.
As set out in Document 8 below, Tretter showed that a statistically efficient estimator of the frequency of a noisy sinusoid may be generated using linear regression techniques. An alternative CRB-achieving scheme, proposed by Kay (as set out in Document 3 below), exploits the correlation between the phase of the incoming samples. Later, Luise and Reggiannini (as set out in Document 5 below) used maximum likelihood techniques to derive a cost-effective frequency recovery algorithm for high-SNR signals. One other notable estimator is that proposed by Mengali and Morelli in Document 7 below, which has the advantage of a particularly large estimation range.
The amount of published work in the area of frequency estimation specifically for DOCSIS upstream channels is relatively limited. As set out in Document 10 below, Wang and Speidel suggested a technique based upon the measurement of the phase angle of the output of a preamble detector. Most recently, Kim et al. (as set out in Document 4 below) surveyed the literature and concluded that Mengali and Morelli's technique is well-suited to the upstream channel.
Despite all of these results, DOCSIS upstream channels present one key issue which does not appear to have been given much attention in the literature: micro-reflections or ‘echoes’ (as set out in Document 1 below). Improperly terminated CMs in a DOCSIS network often reflect multiple copies of the transmitted upstream signals back to the CMTS. The result is significant intersymbol interference (151) in the received signal, which tends to have a biasing effect upon the previously discussed carrier frequency estimators.
The following documents provide further information on this subject:
[1] CableLabs. Data Over Cable Service Interface Specifications DOCSIS 3.0-Physical Layer Specification. CM-SP-PHYv3.0-103-070223, 2007.
[2] Golomb, S. and Scholtz, R. Generalized Barker sequences. Information Theory, IEEE Transactions on, 11(4):533-537, 1965.
[3] Kay, S. A fast and accurate single frequency estimator. Acoustics, Speech and Signal Processing, IEEE Transactions on, 37(12):1987-1990, 1989.
[4] Youngje Kim and Hyunju Ha and Junseo Lee and Wangrok Oh and Whanwoo Kim and Eungdon Lee and Yun-Jeong Song. Upstream Channel Synchronization for Cable Modem System. Advanced Communication Technology, The 9th International Conference on, pages 1864-1867, 2007.
[5] Luise, M. and Reggiannini, R. Carrier frequency recovery in all-digital modems for burst-mode transmissions. Communications, IEEE Transactions on, 43(234):1169-1178, 1995.
[6] Mengali, U. and Morelli, M. Data-aided frequency estimation for burst digital transmission. Communications, IEEE Transactions on, 45(1):23-25, 1997.
[7] Rife, D. and Boorstyn, R. Single tone parameter estimation from discrete-time observations. Information Theory, IEEE Transactions on, 20(5):591-598, 1974.
[8] Tretter, S. Estimating the frequency of a noisy sinusoid by linear regression (Corresp.). Information Theory, IEEE Transactions on, 31(6):832-835, 1985.
[9] Volder, Jack E. The CORDIC Trigonometric Computing Technique. Electronic Computers, IEEE Transactions on, EC-8(3):330-334, 1959.
[10] Jianxin Wang and Speidel, J. Packet acquisition in upstream transmission of the DOCSIS standard. Broadcasting, IEEE Transactions on, 49(1):26-31, 2003.
[11] Berscheid, B., et al., Signal Equalizer for a Signal Transmission Network, U.S. patent application Ser. No. 12/815,611 filed Jun. 15, 2010.
All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. The disclosures in the above documents can be considered for further details of any matters not fully discussed herein.